Methods of forming SOI insulator layers and methods of forming transistor devices

ABSTRACT

The invention encompasses methods of forming individual silicon-on-insulator layers having varying thicknesses within the individual layers. The invention also encompasses methods of forming transistor devices from silicon-on-insulator layers. Additionally, the invention encompasses semiconductor devices and assemblies utilizing silicon-on-insulator layers. The invention includes a method comprising: a) providing a substrate; b) providing an insulator layer over the substrate; c) providing a semiconductive layer over the insulator layer, the semiconductive layer having a first portion and a second portion; d) forming a depletion region within the semiconductive layer and proximate the insulator layer, the depletion region having a different thickness in the first portion than in the second portion; and f) etching the semiconductive layer to about the depletion region. The invention also includes a method comprising: a) providing a semiconductive substrate; b) forming a conductivity-modifying diffusion region in only a portion of the substrate; c) forming an insulator layer over the semiconductive substrate; d) forming a semiconductive layer over the insulator layer; e) forming a depletion region within the semiconductive layer, the depletion region being proximate the insulator layer and having a different thickness over the conductivity-modifying diffusion region than over other portions of the substrate; and f) etching the semiconductive layer to about the depletion region.

RELATED PATENT DATA

This patent resulted from a continuation application of U.S. patentapplication Ser. No. 08/916,773, which was filed on Aug. 20, 1997, nowU.S. Pat. No. 5,940,691.

TECHNICAL FIELD

The invention pertains to methods of forming silicon-on-insulatorlayers, methods of forming transistor devices, and to semiconductordevices and assemblies.

BACKGROUND OF THE INVENTION

Numerous semiconductor devices and assemblies may be formed utilizingsilicon-on-insulator (SOI) constructions. Such assemblies can include,for example, fully depleted SOI devices or partially-deleted SOIdevices. Among SOI devices are n-channel transistors and p-channeltransistors. Such transistors can, depending on the desiredcharacteristics, be either fully depleted SOI devices or partiallydepleted SOI devices. Also, such transistors can be incorporated intospecific types of devices, such as, for example, memory array transistordevices and peripheral transistor devices.

There are generally two ways of providing a starting substrate for SOIfabrication. In a first method, oxygen is implanted at a desired depthinto a silicon wafer. The wafer is then subjected to an anneal to form aburied silicon dioxide layer having an outward monocrystalline siliconlayer thereover. The anneal can also repair damage caused by theimplant, although the repair is typically not perfect.

In a second method, a silicon wafer is initially provided with an outersilicon dioxide layer. Such outer silicon dioxide layer can be formed,for example, by exposing the wafer to an oxidizing ambient. Afterformation of the outer silicon dioxide layer, a separate silicon waferis positioned against the silicon dioxide layer to form a compositecomprising the silicon dioxide layer sandwiched between a pair ofsilicon wafers. The composite is heated in a furnace to cause fusing ofthe silicon wafers with the silicon dioxide. Thereafter, the secondsilicon wafer is mechanically polished down to a desired thickness suchthat its remnants constitute an SOI construction.

In many applications, it is desired to have an SOI construction in whichthe silicon layer has a substantially uniform thickness throughout itsconstruction. A method for improving the uniformity of thickness of asilicon layer in an SOI construction is described with reference to FIG.1, which illustrates an apparatus 10 configured for electrostaticallyetching a silicon layer of a SOI construction. Apparatus 10 comprises avessel 12 within which is an etching composition 14 preferablycomprising potassium hydroxide. A heater 16 is provided within etchingcomposition 14 to control a temperature of the composition during anetching process. An SOI construction 18 is supported within vessel 12 bya TEFLON™ holder 20 which comprises a back support 22 and a frontsupport 24. SOI construction 18 is compressed between back support 22and front support 24. A first O-ring 26 is between SOI construction 18and front support 24 and seals a back of SOI construction 18 fromexposure to etching composition 14. A second O-ring 34 seals frontsupport 24 against back support 22. An electrode 28 extends across aback of SOI construction 18 and supports the back of SOI construction 18while also providing an electrical connection to SOI construction 18.Electrode 28 is electrically connected through a voltage supply 30 to aplatinum electrode 32 extending within etching composition 14.

An expanded view of zone 2 of FIG. 1 is shown in FIG. 2. As shown, SOIconstruction 18 comprises a substrate portion 40, an insulator layer 42,and a silicon layer 44. Silicon layer 44 is a “frontside” ofconstruction 18 and substrate 40 is a “backside” of construction 18.Frontside 44 is exposed to etching composition 14.

In operation, silicon layer 44 is generally lightly doped, with, forexample, a p-type conductivity-enhancing dopant. A voltage is providedwith voltage supply 30 to force a depletion region 46 to be formedwithin silicon layer 44. Etching composition 14 then etches siliconlayer 44 to about depletion region 46 and stops. A thickness ofdepletion region 46 can be controlled by controlling a voltage providedby voltage supply 30. Although it is not clear if the etchingcomposition stops etching at the depletion layer, or at some locationnear the depletion layer, it is clear that the amount of silicon etchedfrom layer 44 can be controlled by controlling a thickness of depletionlayer 46.

In operation, a 20% (wt) potassium hydroxide solution is typically usedas an etchant and 50-75 volts are applied by voltage supply 30 for atypical etching duration of about six minutes. The temperature of thepotassium hydroxide solution is typically controlled to be about 70° C.with temperature controller 16.

The above-discussed methods of forming SOI constructions are utilized toform constructions in which the silicon layer has a relatively uniformthickness. However, in accordance with the invention which follows it isrecognized that there may be some applications in which it is desirableto form SOI constructions having a silicon layer of varying thickness.Accordingly, methods are described for creating SOI constructions inwhich the silicon layer has a varying thickness. Also described areassemblies and devices designed to take advantage of an SOI constructionwithin which the silicon layer has a varied thickness.

SUMMARY OF THE INVENTION

The invention encompasses methods of forming SOI constructions havingvarying thicknesses within the silicon layer. The invention alsoencompasses methods of forming transistor devices from such SOIconstructions. Additionally, the invention encompasses semiconductordevices and assemblies utilizing SOI constructions which have varyingthicknesses of the silicon layer.

In one aspect, the invention encompasses a method of forming asemiconductor-on-insulator layer wherein a substrate is provided, aninsulator layer is provided over the substrate and a semiconductivelayer is provided over the insulator layer. The semiconductive layer hasa first portion and a second portion. A depletion region is formedwithin the semiconductive layer proximate the insulator layer. Athickness of the depletion region is controlled to form a differentthickness in the first portion than in the second portion. Thesemiconductive layer is etched to about the depletion region.

In another aspect, the invention encompasses a method of forming asemiconductor-on-insulator layer wherein a semiconductive substrate isprovided. The semiconductive substrate has a substantially uniformdoping with a first impurity. A first portion of the semiconductivesubstrate is doped with a second impurity. A second portion of thesemiconductive substrate is doped with a third impurity. A third portionof the semiconductive substrate is left undoped with either of thesecond impurity or the third impurity. An insulator layer is formed overthe semiconductive substrate. A semiconductive layer is formed over theinsulator layer. The semiconductive layer is substantially uniformlydoped with a p-type impurity. A depletion region is formed within thesemiconductive layer. The depletion region is formed over the firstportion, the second portion and the third portion. The depletion regionis thicker over the first portion of the substrate relative to over thesecond and third portions. The depletion region is thinner over thesecond portion of the substrate relative to over the first and thirdportions. The semiconductive layer is exposed to an electrolytic etchingcomposition to etch the semiconductive layer to about the depletionregion.

In another aspect, the invention encompasses a method of forming a thinfilm transistor wherein a substrate is provided. A source templateportion, a channel template portion and a drain template portion of thesubstrate are defined. The source and drain template portions are dopeddifferently than the channel template portion. An insulator layer isformed over the substrate. A semiconductive layer is formed over theinsulator layer. A depletion region is formed within the semiconductivelayer. The depletion region is over the source template portion, thechannel template portion and the drain template portion. The depletionregion is proximate the insulator layer, and is thinner over the channeltemplate portion than over the source and drain template portions. Thesemiconductive layer is exposed to an electrolytic etching compositionto etch the semiconductive layer to about the depletion region. Theetched semiconductive layer is thinner over the channel template portionthan over the source and drain template portions. The etchedsemiconductive layer over the channel template portion comprises atransistor channel, the etched semiconductive layer over the sourcetemplate portion comprises a transistor source, and the etchedsemiconductive layer over the drain template portion comprises atransistor drain. A transistor gate is formed proximate the transistorchannel. The transistor source, transistor drain, transistor channel andtransistor gate are incorporated into a thin film transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a diagrammatic cross-sectional view of a prior art apparatusfor electrostatically etching a silicon layer of an SOI construction.

FIG. 2 is an expanded view of the zone labeled 2 in FIG. 1.

FIG. 3 is a fragmentary cross-sectional view of a silicon-on-insulatorconstruction shown at a preliminary step of a first embodiment processof the present invention, and shown in an expanded view similar to thatof FIG. 2.

FIG. 4 is a view of the FIG. 3 silicon-on-insulator construction shownat a processing step subsequent to that of FIG. 3.

FIG. 5 is a fragmentary cross-sectional view of a silicon-on-insulatorconstruction shown at a preliminary step of a second embodiment processof the present invention, and shown in an expanded view similar to thatof FIG. 2.

FIG. 6 is a fragmentary cross-sectional view of a silicon-on-insulatorconstruction shown at a preliminary step of a third embodiment processof the present invention, and shown in an expanded view similar to thatof FIG. 2.

FIG. 7 is a fragmentary cross-sectional view of a silicon-on-insulatorconstruction at a preliminary step of a fourth embodiment process of thepresent invention.

FIG. 8 is a fragmentary cross-sectional view of the FIG. 7silicon-on-insulator construction shown at processing step subsequent tothat of FIG. 7.

FIG. 9 is a fragmentary cross-sectional view of a semiconductor assemblyformed according to a fifth embodiment method of the present invention.

FIG. 10 is a fragmentary cross-sectional view of a semiconductorassembly formed according to a sixth embodiment method of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

The invention generally encompasses methods of controlling a thicknessof a semiconductor layer in a semiconductor-on-insulator assembly,encompasses methods of incorporation of such SOI assemblies intocircuitry devices, and encompasses circuitry devices incorporating suchSOI assemblies.

A first embodiment method of the present invention is discussed withreference to FIGS. 3 and 4. Referring to FIG. 3, an SOI assembly 50 isillustrated. SOI assembly 50 is illustrated in an expanded view similarto that of FIG. 2, with SOI construction 50 being treated withelectrolytic etching apparatus 10 of FIG. 1. SOI construction 50comprises a substrate 52, an insulator layer 54 and a semiconductivelayer 56 over insulator layer 54. Semiconductive layer 56 preferablycomprises a semiconductive material, such as silicon doped with a p-typeconductivity enhancing dopant. Insulator layer 54 can comprise a numberof materials known to persons of ordinary skill in the art, including,for example, silicon dioxide and silicon nitride.

An electrode 28 is electrically connected with substrate 52 and anetching composition 14 is adjacent an outer surface of semiconductivelayer 56. Electrode 28 is connected to a voltage supply 30 (shown inFIG. 1) which is in turn connected to an electrode 32 (shown in FIG. 1).Electrode 32 can be a platinum electrode, or can comprise otherelectrode materials. Voltage supply 30 (shown in FIG. 1) is utilized toform a depletion region 58 (shown in dashed line) within semiconductivelayer 56.

SOI construction 50 comprises three portions 60, 62 and 64, havingdiffering thicknesses of depletion region 58. The differing thickness ofdepletion region 58 in portions 60, 62 and 64 is caused by formingdiffering electric fields in portions 60, 62 and 64. The shown firstembodiment method of controlling the thickness of depletion region 58encompasses providing doped regions 66 and 68 within substrate 52 todefine portions 60 and 64, while portion 62 is defined by the uniformdopant concentration within substrate 52.

In the shown embodiment, substrate 52 comprises a semiconductivematerial, such as silicon, which is uniformly conductively doped to fromabout 10¹⁵ to about 10¹⁸ atoms/cm³ with either a p-type or an n-typeconductivity enhancing dopant. Doped regions 66 and 68 are provided withconductivity enhancing dopant which is either a different type from thatutilized to uniformly dope substrate 52, or which is at a differentconcentration from the uniform substrate doping of substrate 52. Forinstance, if substrate 52 is blanket doped with a p-type conductivityenhancing dopant to a concentration of from 10¹⁵ to about 10¹⁸atoms/cm³, region 66 could comprise a p-type conductivity enhancingdopant provided to a concentration of from about 10¹⁸ to about 10²⁰atoms/cm³, and more preferably from about 10¹⁸ to about 10¹⁹ atoms/cm³.Region 66 will comprise a higher concentration of p-type conductivityenhancing dopant than the portion of substrate 52 within portion 62. Thehigher concentration of p-type conductivity enhancing dopant withinregion 66 relative to the substrate 52 within portion 62 causesdepletion region 58 to be thicker within portion 60 than within portion62.

Dopant region 68 can comprise an n-type conductivity enhancing dopantprovided to a concentration of from 10¹⁷ to about 10²⁰ atoms/cm³. Then-type dopant of region 68 causes depletion region 58 to be thinnerwithin portion 64 than within portions 60 and 62.

Doped regions 66 and 68 can alternatively be referred to asconductivity-modifying diffusion regions 66 and 68.Conductivity-modifying diffusion regions 66 and 68 alter a conductivityinduced by voltage supply 30 (shown in FIG. 1) within semiconductivelayer 56 and thereby modify the thickness of depletion region 58 withinportions 60 and 64.

An alternate way of viewing the FIG. 3 construction is that apredominate portion of depletion region 58 comprises a base thickness“X” defined by the thickness within portion 62. Depletion region 58comprises stepped segments at portions 60 and 64 which have thicknesses“Y” and “Z” that are different from base thickness “X”.

Referring to FIG. 4, SOI construction 50 is shown after etchingcomposition 14 (shown in FIG. 3) etches to about depletion region 58(shown in FIG. 3) to form the illustrated undulating outer surface 70 onsemiconductive layer 56, and after SOI construction 50 is removed fromapparatus 10 (shown in FIG. 1).

A second embodiment method for controlling a thickness of a depletionregion within an SOI assembly is described with reference to FIG. 5. Inreferring to FIG. 5, similar labeling will be utilized as was used indescribing the first embodiment of FIG. 3, with differences beingindicated by the suffix “a,” or by different numerals.

Referring to FIG. 5, an SOI construction fragment 50 a is illustrated.Fragment 50 a is shown in an expanded view similar to the views of FIGS.2 and 3, and is within an apparatus 10 (shown in FIG. 1). SOIconstruction 50 a comprises a substrate 52 a, an insulator layer 54 aand a semiconductive material layer 56 a. Substrate 52 a preferablycomprises a semiconductive material conductively doped with eitherp-type conductivity enhancing dopant or with n-type conductivityenhancing dopant. Semiconductive material 56 a preferably comprises amaterial, such as silicon, conductively doped with p-type conductivityenhancing dopant.

SOI construction 50 a comprises three portions 60 a, 62 a and 64 a, eachdefined by a differing thickness of depletion region 58 a.

Insulator layer 54 a is modified to form regions of varying insulatorthickness and thereby to form the differing thicknesses of portions 60a, 62 a and 64 a. Insulator layer 54 a comprises a first material 80 atportion 62 a and a second material 82, which is different from firstmaterial 80, at portion 60 a. In the shown embodiment, second material82 has a higher dielectric constant than first material 80. Such couldbe accomplished by, for example, utilizing silicon nitride as secondmaterial 82 and silicon oxide as first material 80. The high dielectricconstant of material 82 causes depletion region 58 a to be thicker atportion 60 a than at portion 62 a.

Insulator layer 54 a is thickened at portion 64 a more than at portion62 a, and is displaced inwardly into substrate 52 a at portion 64 arelative to portion 62 a. Such thickening and inward displacement oflayer 54 a can be accomplished by, for example, forming a trench 84within substrate 52 a at portion 64 a and subsequently filling trench 84with insulator layer 54 a. The thickening and inward displacement ofinsulator layer 54 a within portion 64 a, relative to portion 62 a,causes depletion region 58 a to be thinner at portion 64 a relative toportion 62 a.

After formation of depletion region 58 a, electrolytic etching component14 (shown in FIG. 1) can be utilized to etch to about depletion region58 a to form an SOI construction having an undulating outer surfaceshaped similar to depletion region 58 a.

Referring to FIG. 6, a third embodiment of the present invention isillustrated. In referring to FIG. 6, similar labels to those utilizedabove with respect to FIGS. 3-5 will be used, with differences indicatedwith the suffix “b” or with different numerals. FIG. 6 shows an SOIconstruction 50 b within an apparatus 10 (shown in FIG. 1), in a viewsimilar to that of FIGS. 2, 3 and 5. Construction 50 b comprises asubstrate 52 b, an insulator layer 54 b, and a semiconductive materiallayer 56 b. Substrate 52 b preferably comprises a uniformly dopedsemiconductive substrate, insulator layer 54 b preferably comprises aninsulative material such as silicon nitride or silicon dioxide, andsemiconductive material 56 b preferably comprises a p-type doped siliconmaterial.

A conductive material 90 is provided within insulative material 54 b.Conductive material 90 changes an electric field within portion 60 b andthereby causes depletion region 58 b to have a different thicknesswithin portion 60 b than within portion 62 b. Conductive material 90could comprise, for example, a metallic material, such as titanium ortungsten. Although conductive material 90 is shown within insulatorlayer 54 b, it will be recognized by persons of ordinary skill in theart that conductive material 90 could also be provided within substrate52 b to achieve a similar effect on depletion region 58 b as thatillustrated.

SOI assembly 50 b also comprises a trench 84 b, similar to trench 84 inFIG. 5. Trench 84 b causes a thickness of depletion region 58 b to bealtered within region 64 b relative to the thickness of depletion region58 b within, for example, region 62 b.

After formation of depletion region 58 b, the electrolytic etchingcomponent 14 (shown in FIG. 1) can be utilized to etch to aboutdepletion region 58 b to form an SOI construction having an undulatingouter surface shaped similar to depletion region 58 b.

The SOI assemblies produced by the methods discussed above withreference to FIGS. 3-6 may be utilized to form semiconductor devices.For instance, a thin film transistor may be formed according to aprocess illustrated in FIGS. 7 and 8. In referring to FIGS. 7 and 8,similar labels to those utilized above with reference to FIGS. 3-6 willbe utilized, with differences indicated by the suffix “c” or withdifferent numerals.

Referring to FIG. 7, an SOI assembly 50 c is illustrated. Assembly 50 ccomprises a substrate 52 c, an insulator layer 54 c, and asemiconductive material layer 56 c. A source template region 100, achannel template region 102, and a drain template region 104 are definedwithin assembly 50 c. A depletion region 58 c is formed withinsemiconductive layer 56 c. Depletion region 58 c can be formed by one ormore of the methods discussed above with reference to FIGS. 3-6.Depletion region 58 c is formed over source template region 100, channeltemplate region 102 and drain template region 104. Depletion region 58 cis proximate insulator layer 54 c, and has a different thickness withinchannel template region 102 than within source and drain templateregions 100 and 104.

While the depletion region 58 c is formed, semiconductive layer 56 c isexposed to an electrolytic etching composition (such as composition 14shown in FIG. 1) and etched to about depletion region 58 c.

Referring to FIG. 8, SOI assembly 50 c is shown after such etching. Asshown, the etched semiconductive layer 56 c comprises a differentthickness within channel template region 102 than within source anddrain template regions 100 and 104. In the shown embodiment,semiconductive material 56 c is thinner within channel template region102 than within source and drain template regions 100 and 104. However,as will be recognized by persons of ordinary skill in the art,alternative embodiments could be utilized in which the thickness ofinsulator layer 56 c is greater within channel template region 102 thanwithin source and drain template regions 100 and 104.

The etched semiconductive layer 56 c within source and drain templateregions 100 and 104 is then conductively doped to form source and drainregions 106 and 108, respectively. Also, a transistor gate 110 is formedover the etched semiconductive material 56 c within channel templateregion 102 to utilize the material 56 c within region 102 as atransistor channel 112. Channel 112, transistor gate 110, source region106 and drain region 108 together comprise a transistor device 114.

A thickness of channel 112 is controlled by controlling a thickness ofregion 58 c within channel template region 102. Such control of thethickness of channel 112 can be utilized to create a desired thresholdvoltage for transistor device 114.

Referring to FIG. 9, a method of the present invention can be utilizedto selectively adjust thicknesses of peripheral n-channel transistordevices, peripheral p-channel transistor devices, and memory arraydevices, formed on SOI assemblies. In referring to FIG. 9 similar labelsto those utilized above in describing FIGS. 1-8 will be used, withdifferences indicated by the suffix “d” or different numerals.

FIG. 9 illustrates an SOI assembly 50 d comprising a substrate 52 d, aninsulator layer 54 d and a semiconductive layer 56 d. Within SOIassembly 50 d are defined a peripheral p-channel device region 120, aperipheral n-channel device region 122, and a memory array device region124. Ultimately, p-channel transistors (not shown) will be formed withinregion 120, n-channel transistors (not shown) will be formed withinregion 122, and memory array transistors (not shown) will be formedwithin region 124. The methods discussed above regarding FIGS. 3-8 canbe utilized to adjust relative thicknesses of semiconductive material 56d in the regions 120, 122 and 124 to enable the ultimately formedp-channel devices, n-channel devices and memory array devices to beformed over regions of semiconductive material 56 d with locally varyingthicknesses of channel regions and/or source/drain regions. In fullydepleted SOI a threshold voltage can be influenced by a silicon layerthickness, as well as by variations of doping within the silicon layer.Thus, local variation of thickness of the semiconductive layer 56 dwithin regions 120, 122 and 124 can be utilized to adjust a thresholdvoltage of the peripheral devices formed in regions 120 and 122 to bedifferent from a threshold voltage of the memory array devices formed inregion 124.

Referring to FIG. 10, a method of the present invention can be utilizedto form SOI assemblies in which a semiconductive layer has a differentthickness in a fully depleted SOI region than in a partially depletedSOI region. Generally, the semiconductive material would be thicker in apartially depleted SOI region than in a fully depleted SOI region,although, as will be recognized by persons of ordinary skill in the art,methods of the present invention can be utilized to form oppositestructures as well.

In referring to FIG. 10, similar labels to those utilized in discussingFIGS. 3-9 above will be utilized, with differences indicated by thesuffix “e” or by different numerals.

FIG. 10 illustrates an SOI assembly 50 e comprising a substrate 52 e, aninsulator layer 54 e, and a semiconductive layer 56 e. Within SOIassembly 50 e are defined partially depleted SOI template regions 130and fully depleted SOI template regions 132. Semiconductive material 56e is thicker within the partially depleted SOI regions 130 than withinfully depleted SOI regions 132. Such variation in the thickness ofsemiconductive material 56 e can be accomplished by the methodsdiscussed above regarding FIGS. 3-6.

To aid in interpretation of the claims that follow, the term“semiconductive substrate” is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove.

Although the embodiments discussed above are described with reference toformation of SOI structures utilizing the apparatus of FIG. 1, it is tobe understood that other apparatuses could be utilized in methods of thepresent invention.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. A method of patterning a semiconductive material,comprising: forming a depletion region within the semiconductivematerial, the depletion region having a different thickness in oneportion of the semiconductive material than in an other portion; andetching the semiconductive material to about the depletion region.
 2. Amethod of forming a semiconductor-on-insulator layer comprising:providing a substrate having an insulative material and a semiconductivematerial thereover, the insulative material being between the substrateand the semiconductive material; forming a depletion region within thesemiconductive material, the depletion region having a differentthickness in one portion of the semiconductive material than in an otherportion; and etching the semiconductive material to about the depletionregion.
 3. The method of claim 2 wherein the substrate beneath said oneportion has a different conductivity than the substrate beneath saidother portion.
 4. The method of claim 2 wherein the etching comprisesutilizing a potassium hydroxide solution to etch the semiconductivelayer.
 5. The method of claim 2 wherein the substrate is semiconductiveand wherein the substrate beneath said one portion is doped differentlythan the substrate beneath said other portion.
 6. The method of claim 2wherein a trench is provided beneath said one portion and not beneathsaid other portion.
 7. The method of claim 2 wherein a metal-comprisinglayer is provided beneath said one portion and not beneath said otherportion.
 8. The method of claim 2 wherein the insulator layer is formedof two materials, the two materials comprising a first material whichunderlies said one portion and a second material which underlies saidother portion.
 9. The method of claim 8 wherein the first materialcomprises silicon nitride and wherein the second material comprisessilicon oxide.
 10. A method of forming a semiconductor-on-insulatorlayer, comprising: providing a substrate; providing an insulator layerover the substrate; providing a semiconductive layer over the insulatorlayer, the semiconductive layer having a first portion and a secondportion; forming a depletion region within the semiconductive layer, thedepletion region varying in thickness within the semiconductive layer;and etching the semiconductive layer to about the depletion region. 11.A method of patterning a semiconductive material, comprising: providingan insulative mass comprising a first insulative material and a secondinsulative material; providing a semiconductive material over theinsulative mass; forming a depletion region within the semiconductivematerial, the depletion region having a different thickness over thefirst insulative material than over the second insulative material; andetching the semiconductive material to about the depletion region. 12.The method of claim 11 wherein the insulative first material comprisessilicon nitride and the insulative second material comprises siliconoxide.
 13. A method of forming a semiconductor-on-insulator layercomprising: providing a semiconductive substrate having substantiallyuniform doping with a first impurity; doping at least one portion of thesemiconductive substrate with a second impurity while leaving at leastone other portion of the semiconductive substrate not doped with thesecond impurity; forming an insulative material over the semiconductivesubstrate; forming a semiconductive layer over the insulative material;forming a depletion region within the semiconductive layer, thedepletion having a different thickness over the first portion of thesubstrate than over the second portion of the substrate; and etching thesemiconductive layer to about the depletion region.
 14. The method ofclaim 13 wherein the first impurity comprises a p-type dopant.
 15. Themethod of claim 13 wherein the first impurity comprises an n-typedopant.
 16. The method of claim 13 wherein the first impurity and thesecond impurity comprise a common conductivity type.
 17. The method ofclaim 13 wherein the first impurity comprises a first conductivity-typedopant and wherein the second impurity comprises a secondconductivity-type dopant, the second conductivity type being differentthan the first conductivity type.
 18. The method of claim 13 wherein thefirst impurity and the second impurity comprise a common dopant.
 19. Amethod of forming a semiconductor-on-insulator layer comprising:providing a semiconductive substrate having substantially uniform dopingwith a first impurity of a first conductivity type; doping a firstportion of the semiconductive substrate with a second impurity havingthe first conductivity type; doping a second portion of thesemiconductive substrate with a third impurity having a secondconductivity type; leaving a third portion of the semiconductivesubstrate undoped with either of the second impurity or the thirdimpurity; forming an insulator layer over the semiconductive substrate;forming a semiconductive layer over the insulator layer, thesemiconductive layer comprising silicon; substantially uniformly dopingthe semiconductive layer with a p-type impurity; forming a depletionregion within the semiconductive layer, the depletion region being overthe first portion, the second portion, and the third portion, thedepletion region being proximate the insulator layer and having adifferent thickness over the first portion of the substrate than overthe second and third portions, the depletion region having a differentthickness over the second portion of the substrate than over the firstand third portions; and while the depletion region is formed, exposingthe semiconductive layer to an electrolytic etching composition to etchthe semiconductive layer to about the depletion region.
 20. The methodof claim 19 wherein the first conductivity type is p-type and the secondconductivity type is n-type.
 21. The method of claim 19 wherein thefirst conductivity type is n-type and the second conductivity type isp-type.
 22. The method of claim 19 wherein the first impurity and thesecond impurity comprise a common dopant.
 23. A method of forming asemiconductor-on-insulator layer comprising: providing a semiconductivesubstrate having substantially uniform doping with a first impurity;doping a first portion of the semiconductive substrate with a secondimpurity; doping a second portion of the semiconductive substrate with athird impurity; leaving a third portion of the semiconductive substrateundoped with either of the second impurity or the third impurity;forming an insulator layer over the semiconductive substrate; forming asemiconductive layer over the insulator layer, the semiconductive layercomprising silicon; substantially uniformly doping the semiconductivelayer with a p-type impurity; forming a depletion region within thesemiconductive layer, the depletion region being over the first portion,the second portion and the third portion, the depletion region beingthicker over the first portion of the substrate relative to over thesecond and third portions, the depletion region being thinner over thesecond portion of the substrate relative to over the first and thirdportions; and while the depletion region is formed, exposing thesemiconductive layer to an electrolytic etching composition to etch thesemiconductive layer to about the depletion region.
 24. The method ofclaim 23 wherein the first and second impurities comprise ap-conductivity type, and wherein the third impurity comprises ann-conductivity type.
 25. The method of claim 23 wherein the firstimpurity and the second impurity comprise a common conductivity type.26. The method of claim 23 wherein the first impurity and the secondimpurity comprise p-conductivity type.
 27. The method of claim 23wherein the first impurity and the second impurity comprise a commondopant.
 28. A semiconductor processing method of forming fully depletedSOI and partially depleted SOI on the same wafer comprising: providing asubstrate; defining fully depleted SOI template portions and partiallydepleted SOI template portions of the substrate; forming an insulatorlayer over the substrate; forming a semiconductive layer over theinsulator layer, the semiconductive layer comprising silicon; forming adepletion region within the semiconductive layer, the depletion regionbeing over the partially depleted SOI template portion and over thefully depleted SOI template portion, the depletion region beingproximate the insulator layer and being thicker over the fully depletedSOI template portion than over the partially depleted SOI templateportion; while the depletion region is formed, exposing thesemiconductive layer to an electrolytic etching composition to etch thesemiconductive layer to about the depletion region, the etchedsemiconductive layer comprising a greater thickness over the fullydepleted SOI template portion than over the partially depleted SOItemplate portion; forming a fully depleted SOI from the etchedsemiconductive layer over the fully depleted SOI template portion; andforming a partially depleted SOI from the etched semiconductive layerover the partially depleted SOI template portion.
 29. The method ofclaim 28 wherein the substrate is a semiconductive substrate, the methodfurther comprising; before forming the depletion region, doping thefully depleted SOI template portion of the semiconductive substratedifferently than the partially depleted SOI template portion of thesemiconductive substrate.